1. Field of the Invention
The present invention relates generally to non-invasive hemodynamic monitoring and, more particularly, to a method and apparatus for detection of sudden blood loss by monitoring and processing of photoplethysmography (PPG) data.
2. Background of the Related Art
Real-time, accurate warning of impending hypovolemia remains elusive, and vital signs often do not substantially change between the times of hemorrhage until cardiovascular collapse. Conventional devices use a PPG signal to non-invasively detect blood and fluid loss. Conventional systems also observe respiratory-induced variations in PPG signal amplitude for exaggerations that might relate to blood volume loss in mechanically ventilated patients. However, the relevance, sensitivity and specificity of respiratory variation in PPG amplitude in awake, spontaneously breathing patients and healthy subjects has yet to be established.
Pulse oximetry is a commonly used noninvasive technique to continuously monitor arterial oxygen saturation (SpO2) and Heart Rate (HR). A PPG sensor that comprises the pulse oximeter reflects changes in the light absorption of a vascular bed containing a pulsatile, arterial component, and a slowly varying venous component, related to the average blood volume and tissue properties.
Hemodynamic response to progressive acute hypovolemia is a complex process with two distinct phases. In a first phase, an arterial baroreceptor-mediated phase, a fall in cardiac preload is nearly matched by a sympathetically mediated increase in peripheral resistance so that arterial Blood Pressure (BP) is maintained near normal levels. When blood volume falls to a critical level, i.e. approximately thirty percent of a normal level, a second phase abruptly develops. The second phase is characterized by withdrawal of sympathetic vasoconstrictor drive, relative or absolute bradycardia, and a profound fall in arterial pressure. Blood flow to the brain and heart is compromises, posing a serious threat to life. Therefore, early detection of blood loss, such as from hemorrhage, remains a challenging task in emergency/critical care medicine, surgery and anesthesia, given the opacity of routinely measured vital signs such as HR and BP which reflect the symptoms of blood loss only after at least 30% of circulating blood volume is lost.
A major impediment in early detection of blood loss of conventional systems is quantifying respiratory variations directly from pulse oximetry. Towards this goal, the Power Spectral Densities (PSD) of the PPG signal has recently been used to calculate the ratio of the respiratory peak power to the heart rate peak power. The present invention identifies an increased ratio that occurs before statistically significant change in BP or HR. Using the PSD is better than time-domain approaches such as Plethysmogram Variability Index (PVI) or Partial Thromboplastin Time (PTT). However, use of PSD is not optimal since time-varying dynamics of the respiratory and heart rates, especially during blood loss, are not accounted for. Also, determining a precise location of the respiratory peak is problematic since the respiratory peak is often one of the smallest peaks in a spectrum, and becomes even more difficult to decipher once motion/noise artifacts appear.
To combat a part of these problems, a time-varying PSD approach has been developed utilizing a short-time Fourier transform (STFT). However, the STFT technique only partially solves the time-varying issue. Further, both the time and frequency resolutions are suboptimal, and not the most effective approach for early blood loss detection. To overcome these shortcoming, a method and apparatus for real-time continuous monitoring of blood volume loss capability is provided herein.